Summary of Work: Metabolism by sulfotransferases is an inactivation of biological signals such as steroid hormones and also a defense system against environmental toxicants and carcinogens. To encounter virtually unlimited numbers of structurally diverse chemiclas, these enzymes exhibit broad substrate specificity. Thus, it is important to investigate the structural principle determining the substrate specificity in order to predict the metabolisms and the resulted consequences to human susceptibililty to chemical exposure. For the investigation, we have used x-ray crystallography and site-directed mutagenesis. We have solved the crystal structure of estrogen sulfotransferase (EST) complexed with PAP and estradiol and that with PAP and vanadate. These structures provided the insight into understanding the reaction mechanism catalyzed by sulfotransferase. Site-directed mutagenesis based on the structres defined the stricture- like gate in the substrate pocket of EST that confers estrogen specificity to the enzyme. - Sulfotranserase, x-ray crystallography, enzyme action, site-directed mutagenesis, protein expression and purification